Conventional multiplexing systems combine several input channels (i.e. data streams) to form a single output data stream, where the total group bit rate of the output data stream is the sum of the bit rates of the constituent parts. However, where the input data streams are the outputs of video or audio compression systems, the sum of their bit rates varies considerably and may regularly exceed the total capacity available in the transmission channel. One way to mitigate this problem is to constrain each channel to a bit rate allocation which is an appropriate share of the total output bit rate, but this requires that the respective channel picture quality is also constrained when the bit rate demand for that channel exceeds the fixed share bit rate allocated to that channel. Meanwhile, other channels may have spare capacity due to their lower instantaneous bit rate demands, but in such fixed share systems the spare capacity is not available for use by other input channels. It is therefore desirable to find means to avoid this quality loss and inefficient use of available output capacity.
A more flexible and effective approach, known as statistical multiplexing, combines a group of encoders to share a fixed total common bit rate, but instead of allocating a fixed bit rate to each encoder, a bit rate controller dynamically allocates different bit rates to each encoder depending on the instantaneous bit rate demand of the respective input video data streams (i.e. channels). The success of this scheme depends on the statistical behaviour of the several encoders and their input video or audio streams. Thus, occasionally, the bit rate demand of some input channels is momentarily lower than the average and so the excess bit rate can be collected in a common pool and re-allocated to supply more bit rate than the average to any channel whose demand is not fully met. As long as the instantaneous total group bit rate is equal to or less than the channel capacity, then no conflict arises between supply and demand. The aim is to allocate available bit rate to the channel encoders according to demand, so that the combined system achieves a more uniform picture quality on each encoder. By sharing out the total group bit rate between the channel encoders, the picture quality of all encoders can be improved compared to the average picture quality of fixed share bit rate systems.
FIG. 1 shows a block diagram of a statistical multiplexing system with three look-ahead encoders 120 and three main encoders 140. The look-ahead encoders 120 provide bit rate estimates to a bit rate controller 130 and these estimates are used by the bit rate controller 130 to allocate appropriate bit rates to the main encoders 140. Delays 160 are used on each channel, between the look-ahead encoders 120 and main encoder 140, to ensure the bit rate controls maintain synchronisation with the respective portion of the input video channels.
Existing statistical multiplexing systems provide single bit rate estimates to the bit rate controller 130. The bit rate controller 130 collects the bit rate estimates from all look-ahead encoders 120 within a statistical multiplexing group and allocates appropriate bit rates to the main encoders 140 dependent on the bit rate estimates.
If the bit rate controller 130 uses a linear allocation algorithm, there is a danger that non-critical channels do not get enough bit rate to achieve the same picture quality as highly critical material.
A similar situation exists if Standard Definition Television (SDTV) channels are statistically multiplexed with High Definition Television (HDTV) channels, since using a linear allocation algorithm would result in badly distorted SDTV channels because the HDTV channels would use up most of the group total bit rate. Therefore, current bit rate distribution methods use a non-linear algorithm to allocate a higher bit rate to non-critical channels relative to critical channels, and to SDTV channels relative to HDTV channels. This goes some way towards equalising picture quality among diverse video signals.
However, there is a disadvantage in using a non-linear distribution algorithm, and that is the bit rate savings of channels using lower resolutions, e.g. ¾ or ½ horizontal resolution, is significantly reduced. Furthermore, sequences with high spatial and/or temporal redundancies such as stills, colour bars or other static test signals end up using too much bit rate (i.e. more than actually required to produce an acceptable result). It is therefore necessary to find better methods and apparatus to manage the bit rate allocations used in statistical multiplexers, in the interests of improving quality for all input video or audio streams, and efficiency of the overall multiplexing system.